<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        input{
            border:0;
            border-bottom: 1px dashed red;
        }
        button{
            width: 50px;
            height: 25px;
            border: 1px solid red;
        }
    </style>
</head>
<body>
    用户名<input type="text">
    <button>按钮</button>
</body>
</html>